Humidifiers are used in a wide variety of applications, including the humidification of gases for use in therapeutic environments or for home heating purposes, as space humidifiers and otherwise and among the humidifiers which may be mentioned are those described in United States Patents: U.S. Pat. Nos. 5,329,939, 5,195,515, 4,891,171, 4,714,078, 4,518,404, 4,500,480, 4,225,542, 4,177,945, 4,100,235, 4,089,915 and others. Among these are those which are self-sterilizing, i.e. reduce the contamination which otherwise tends to develop in a humidifier unit because of the presence of microorganisms in the water which is used or because water at a temperature suitable for the growth of microorganism cultures may stand in the unit.